Download Squatting and the Implications of Technique on Muscle Function

Squatting and the Implications of Technique on Muscle Function
©Copywrite 2003 David Woodhouse
Introduction
The squat has been described as the ‘King of Exercises’ since it activates the largest, most powerful
muscles in the body and is the greatest test of lower body strength (4, 6, 12). The major muscles that
are activated are the ankle, knee and hip extensors, the spinal erectors and the abdominals. As a
result the squat is one of the most popular exercises for development of lower body strength and
power. It constitutes one of the three competitive lifts in the sport of Powerlifting and the front
squat variation is also a component of the ‘Clean’ lift in weightlifting.
The ‘Sticking Point’ Phenomena
When load is near maximal, squat technique may be adjusted to permit a successful completion of
the lift. This results in an asymmetric lift when descending and ascending components are compared
(6). The point of minimal bar velocity during the ascent is often described as the ‘sticking point’.
The sticking point is thought to result from the force-length properties of muscles and the torque
produced by the load (6). The quadriceps’ ability to produce tension decreases as they extend and
hence so does their net extensor moment. At the sticking point, they are no longer able to produce
sufficient force to continue extending the knees (6). Hip flexion at this point occurs to shorten the
load’s moment arm at the knee joints and enables the quadriceps to extend them (17) (and also
lengthen the hamstrings). The vasti muscles of the quadriceps group all show similar peak EMG
activity during ascent and descent (12). The rectus femoris is the only bi-articular muscle in the
quadriceps group, it creates a hip flexor moment and shows ~30% greater activation during the
ascent versus the descent, but still significantly less than the vasti (12). The vasti each have specific
length tension relationships, the vastus medialis is most active during the latter stages of extension
hence it may be a weakness in the vastus lateralis that most contributes to the sticking point (15).
The load’s moment arm at the hip increases with flexion at the sticking point but lengthening of the
gluteal and hamstring muscles is advantageous for producing force since it improves their length
tension relationships and hence increases net torque (6). The hamstrings are a bi-articular muscle,
crossing both knee and hip joints and during the ascent they shorten at the hip and lengthen at the
knees. However, the shortening through hip extension is disproportionate to the lengthening
through knee extension, and therefore their ability to produce tension is improved by hip flexion (6).
Hip flexion increases the moment arm of the load and therefore requires an increase in the isometric
tension produced in the spinal erectors. At the beginning of the ascent some lifters (particularly in
the front squat) hyperextend the spine to shorten the hip’s moment arm and also to help keep the
load-body centre of gravity over the feet (4). Hyperextension of the lumbar spine places greater
stress on the facet joints and may increase the risk of chronic lower back pain (8). Repeated training
in this hyperextended position may cause an exaggeration of the lumbar curve - lordosis (8). At the
sticking point however the spine generally flattens and in some cases may partially flex. This allows
the knees to extend fractionally without any increase in bar height (4).
As the spine flexes, the spinal erector’s length tension relationship moves closer to optimum but the
net extensor torque decreases because there is a significant decrease in the angle the logissimus
thoracis and iliocostalis lumborum muscle fibres make with the spine. This compromises their ability
to support shear forces and hence, at full flexion, those forces are transferred to passive tissues, (i.e.
ligaments and spinal disks) significantly increasing the risk of injury (13). Powerlifters may allow their
spine to flex to within two or three degrees of full flexion hence preventing injury yet maximising the
ability to negotiate the ‘sticking region’ (14). Squatting typically stresses the spinal erectors
isometrically in an extended position. Due to the specificity of this mode of training there is little
cross over to the longer muscle fibre lengths involved in spinal flexion (4). Therefore, if a lifter has
not been conditioned to partially flex the spine, the muscles may not be able to maintain sufficient
tension and it may ‘buckle’ (14).
When squatting lifters employ the Vasalva manoeuvre, that is a voluntary increase in pressurisation
of the abdominal cavity achieved by closing the epiglottis and activating trunk and abdominal
muscles (11). The effect of increasing intra abdominal pressure is increased stability of the spine
though the mechanism is not fully understood (10). An early theory was that a hydrostatic force
within the abdominal cavity induced an extensor moment by pushing down on the pelvic floor and
up on the diaphragm (10). However contraction of rectus abdominis, and the internal/external
oblique muscles causes a flexor moment that offsets the extensor moment caused by intraabdominal pressure (3). It is now believed that increased co-activation of spinal flexor and extensor
muscles increases spinal stiffness and hence spinal stability (3). This means that increased intraabdominal pressure is simply a useful by-product that negates the flexor moment caused by the
abdominal and oblique muscles as discussed above (3).
The changes in technique at the sticking point highlight the influence of load on technique. This is
important when designing protocols to examine the kinematics of the lift. Also the sticking point
phenomena has implications for training to improve squatting strength, for example lifters might
train isometrically at the sticking point to improve strength specifically in that posture (4).
Width of Stance
A wide stance is typical of Powerlifters whilst weightlifters typically use a narrower stance (although
not narrow as defined by the research) Typically in the research a narrow stance was defined to be
~75% of shoulder width whilst wide stance was defined to be ~140% shoulder width. Commonly
lifters show greater lateral rotation of their feet as stance width increases but this variable has not
been shown to influence any muscle activities (6).
Research involving EMG has shown no significant difference in quadriceps activation between
narrow and wide stances (12). Adductor activation has been shown to increase with a wide stance
(12). This is because the thigh shows increased abduction and lateral rotation during the descent
with a wide stance and, during the ascent, the adductors are therefore activated to draw the thigh
back to the midline of the body and also medially rotate it back to a neutral position (12).
Gluteus maximus and hamstring activation also increases with a wide stance (6). It has been
suggested, for the former, that this is due to the positioning of its’ distal attachment, which causes
the gluteus to lengthen with thigh abduction (12). This lengthening shifts it away from its’ optimum
position on the length-tension curve and hence greater activation is required to create the same
tension as the narrower stance (12). Greater activation of hip extensor muscles may also occur
because width stance effects torso inclination (see later).
Narrow stance causes greater forward knee movement and hence greater plantar flexion at the
ankle as the shank inclined (6). This caused an increase in activity of the gastrocnemius during the
ascent phase and since the gastrocnemius is a bi-articular muscle crossing both ankle and knee joints
there is an increased knee flexion moment (6). Intuitively greater quadriceps activity is expected to
counteract this but as discussed above this is not the case. This implies that either this moment is
negligible or that the kinematics are altered. The increased forward knee movement also increases
knee shear force due to more acute angle formed by thigh and shank (6).
These findings, have implications for training for example, for greater development of the adductors
and to minimise shear forces, a wide stance may be preferable. In contrast, a narrower stance may
be more beneficial to increase activation of the gastrocnemius. Powerlifters have found that a wider
stance is one factor that permits them to lift greater loads, however they need only squat to a
position where the thighs break parallel and a wide stance may be less efficient for deeper variations
of the lift.
Bar Position
There are three major methods of supporting the barbell when performing a squat lift. ‘Low bar’
where the bar lies across the spine of the scapula. ‘High bar’, the traditional technique, where the
bar rests on top of the posterior deltoids and trapezius muscles, and ‘Front bar’ where the barbell
rests on the anterior deltoids and clavicals. The load is posterior to the body’s line of gravity with low
and high bar but anterior with front bar. Low bar is the technique utilised by powerlifters, whilst
front bar is used directly in weightlifting during the clean. High bar is the most common technique in
strength training for sport since it has the lowest flexibility and skill requirement.
Whilst there are correlations between bar position and the kinematics of the lift these are subject to
individual differences in technique (16). The major difference is the degree of trunk inclination.
Increased trunk inclination, increases the moment arm at the hip but decreases it at the knee and
(as discussed previously) the greater the moment arm, the greater the muscle activation required to
extend the joint. Bar position also directly influences the moment arm since, at any hip angle, the
distance of the bar to the hip increases from low bar to high bar and again from high bar to front bar
(16).
Typically the front bar causes the most erect trunk posture since over inclination would cause the
bar to fall forward off the chest (4, 17). Since the spine is more resistant to compressive forces
(directed axially) than to shear forces, an erect trunk posture reduces risk of lower back injury (16).
In the high bar the load is typically positioned centrally between knee and hip joints (17). Low bar
allows the greatest hip flexion and hence the shortest moment arm at the knee. This latter bar
position is therefore the most mechanically efficient and hence permits the greatest loads (17).
At the present time there is no research to show whether bar position affects other factors such as
intra-abdominal pressure. There may also be some interaction between other variables such as
stance and depth.
Cadence
Increased lifting speed causes higher maximum, and greater variation in, shear and compressive
force in the knee and spine (9). This is because a faster decent requires greater deceleration forces
from the knee and hip extensors in order to slow and stop the weight at the bottom of the descent
(6). Slower cadences during the descent may therefore be preferred since they decrease risk of
injury and also increase the time under tension hence increasing the training effect (4). Cadence has
not however been shown to significantly effect intra-abdominal pressure (7).
Other factors
There are many other factors, outside the scope of this paper, that influence squat kinematics. For
future work however these include the depth of squat, segmental length, fatigue and equipment
such as shoes, lifting belts and suits.
References
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